Water in The Fault in Our Stars mostly directly represents suffering in both its negative and positive varieties. Water, for instance, symbolizes the fluid that collects in Hazel’s lungs as a result of her cancer. This liquid causes Hazel a huge amount of suffering in the novel. It forces her to use an oxygen tank, limits her ability to do any real strenuous activity, and it nearly kills her at one point. She likens the suffering she feels in that instance to being smashed by waves but unable to drown. (It’s no coincidence that drowning is one of the novel's major motifs, since water is, of course, at its center.) This type of suffering is obviously negative. At the same time, it's significant that Augustus's last name is Waters. He is Hazel's great love in the novel, and his physical deterioration and eventual death cause Hazel an intense amount of pain. Hazel, however, wouldn’t trade that pain for anything. It’s a mark of the love she feels for Augustus, which makes it a kind of positive pain. Hazel even uses the analogy of being smashed by waves but unable to drown again to describe the way she feels after Augustus dies. In doing so she creates a metaphor with two parallel sides: one where drowning in water represents the negative suffering of her cancer, and the other where drowning in water represents the positive suffering of her losing Augustus. Augustus sums up this dual nature of suffering, and water, in the final letter he writes to Van Houten. When he found out Hazel was hospitalized, he snuck into her room in the ICU and found her there unconscious. A nurse told him Hazel was “still taking on water.” Augustus describes this abundance of water to Van Houten as “A desert blessing, an ocean curse.”
It's also worth noting that the two locations in the novel, Indianapolis and Amsterdam, are canal cities. Amsterdam in particular is under constant threat of being inundated by the surrounding waters, like Hazel herself to some degree. It’s also home to Peter Van Houten, whom we learn is drowning, so to speak, in his own suffering over losing his daughter many years earlier to cancer. Finally, the book's epigraph, taken from the metanovel An Imperial Affliction, offers another layer of meaning to water’s symbolism. It refers to water as “conjoinder rejoinder poisoner concealer revelator,” giving it a nearly omnipotent quality, like a god, and compares water to time, both of which take everything with them in their tide.
The cigarettes Augustus often puts in his mouth but doesn't light represent his attempt to deal with and ideally control the things he fears. Though Augustus doesn't say so explicitly, the thing he fears most appears to be cancer. Cigarettes are a well-known carcinogen, and when Augustus explains the cigarette to Hazel it seems it's cancer specifically he is trying to control. Over the course of the novel, however, the cigarettes develop a greater meaning than August initially states. He reaches for them any time he feels insecure, suggesting they act as a symbolic way for him to control all his fears, with cancer just being the most notable. Toward the end of the novel, his incident at the gas station in which he has to call Hazel for help occurs because he's trying to buy cigarettes. In the context of their symbolic value, he is trying to regain control. By this point, his body is failing. He has difficulty walking on his own, he can't fully control his bladder, and when Hazel finds him in his car he's vomited all over himself. He says he just wanted to buy a pack of cigarettes on his own, and the state of his health and the fact that he was unable to get the cigarettes both point to the reality that any control he had over his cancer is gone.
The grenade metaphor signifies death and the suffering a person's death causes to those close to them. Hazel uses the term to describe herself after she reads Caroline Mathers's online profile and sees the effect Caroline's death had on others. She likens herself to a grenade that will one day explode, injuring everyone nearby. She also says Augustus becomes the grenade once his cancer returns and it's evident that he'll die before Hazel does. For Hazel, not hurting others is a major concern. It's evident in her being a vegetarian so that she doesn't add to the suffering in the world, for instance. Knowing the effect her death will have on Augustus and her parents therefore poses a serious conundrum for her. She doesn't want to keep them at a distance, but she feels doing so is the only way to keep them safe. The grenade symbol comes up again and again in this context as she wrestles with her desire to be close to them and her concern that she'll injure them.
But it's also worth noting that the grenade also turns up in the video game Augustus plays with Isaac. In the game, Augustus heroically throws himself on a grenade to save nearby school children in the game. It's only by sacrificing himself and willingly getting hurt by the grenade that Augustus, at least in the game, achieves the heroism he always desires, and in this regard the grenade metaphor ties directly into one of the major lessons in the book. After Augustus dies, Hazel reads a letter he sent to Van Houten in which Augustus discusses the idea of the people close to us hurting us. He says people don't get to choose who they hurt, but they can choose who hurts them. The grenade represents the suffering we cause others, but as Augustus shows in the game, in some cases the cause is worth it. Clearly we see that whatever pain Augustus's death causes Hazel was similarly worth it to her, and what the novel suggests through Augustus's act in the video game is that there's a measure of heroism in being willing to get hurt for the right cause.
An Imperial Affliction has an abundance of metaphorical resonance throughout The Fault in Our Stars. To begin with, it represents the healing value of fiction. Hazel refers to it as her personal bible because it's the only account of living with cancer she's found that corresponds to her own experience. That fact provides her with a great deal of comfort as she battles her illness, and it also establishes the foundation for the novel's other symbolic meaning: It represents Hazel's experience, and in particular her relation to her family. Hazel obsesses over the fates of the characters in the novel because they serve as proxies for her own parents, whom she wants to know will be alright after her death. By learning what happens to them and confirming that they don't simply disappear after Anna's death in the novel, she can feel certain that her parents will similarly live on after her death. The novel doesn't fully elaborate on the idea, but learning their fates might also be her way of confirming that Anna's story, and by extension her own, doesn't just end with her death. If the story continues, then even if Anna is no longer an active presence, she would still be connected to a larger saga that carries on after her passing. If that's true, Hazel could feel she similarly continues to play a part in the larger story of her family and friends, and that she doesn't simply disappear into oblivion with her death.